The present invention relates to devices for the measurement and detection of radiation. More particularly, the present invention relates to liquid ionization chambers having three terminals.
Medical diagnostic procedures frequently rely on imaging systems and sensors to detect and measure radiation used for treatment. It is often necessary to expose a patient to a small amount of radiation after being positioned on a treatment table but before the primary treatment for the purpose of insuring that the patient is correctly positioned for radiation therapy. This is known as localization imaging. During treatment, it is necessary to insure that the patient has not moved and is in the correct position during treatment, and that the appropriate radiation profile is being applied. This is known as verification imaging, and typically consists of a series of individual images of the target area taken throughout the treatment session. Thus, systems used in verification imaging should be capable of the real-time generation of images.
Imaging systems used in these circumstances must be designed for high energy levels. The energy levels used in radiation therapy are generally greater than one million electron volts (MeV) and may typically range from 4 to 25 MeV. In these high radiation situations, it is essential that the treatment be properly directed at the correct treatment area of the patient's body. Further, it is also important that the amount of exposure to other parts of the patient's body be minimized. Thus, for an imaging system to be effective and useful during treatment and treatment planning, it must be suitable for use with high energy radiations, it must be accurate, and it must be able to provide real time images for an entire treatment session.
The most common approach to verification and localization imaging is to record a treatment or treatment sequence on film. Unfortunately however this approach requires delays of minutes or hours to develop the film which can lead to patient discomfort and movement during localization imaging, and cannot provide real time imaging capabilities for verification imaging. Further, the approach is not suitably accurate as patient movement may disturb results of localization imaging, and because the film does not reveal the exact quantity of radiation to which a target area has been exposed.
Another disadvantage of fill imaging systems is that they generally have inadequate resolution and dynamic range. In addition, it is typically not possible to use computer enhancement techniques to improve the image obtained on film. Further, these film systems require a high amount of operator intervention which could result in errors or movement of the patient during treatment. Thus, it would be desirable to provide a system which decreases the amount of operator intervention required and which increases the resolution of images obtained during imaging.
Another device which has been used to perform verification and localization imaging is the scanning two terminal liquid-filled ionization chamber. Examples of these systems are shown, e.g., in U.S. Pat. No. 5,019,711, issued May 28, 1991 to Antonuk, U.S. Pat. No. 5,025,376, issued Jun. 18, 1991 to Bova et al.
Two terminal liquid-filled ionization chambers are typically arranged in a two-dimensional matrix, rows of which are scanned to measure a current at each chamber. These ionization chambers may be regarded as parallel plate capacitors in which the region between plates is led with a liquid. The amplitude of the signal measured is proportional to the number of ions formed (and thus to the energy deposited by the radiation). The radiation intensity is recorded as a current. The ionization current measured is proportional to the energy of the radiation. Thus, higher energy radiation gives more ionization and a greater response.
The current being monitored in such two terminal liquid ion chambers consists of two components: one due to the current flowing to charge the electrode structure and to provide an electric field between the electrodes; and the other component attributable to ion motion in that field. Because only two terminals are used in these devices, the two currents must occur in parallel paths sharing the same terminals. The ion current is the signal current representing the presence of radiation, while the charging current is a transient of the measuring circuit, and must be separated from the signal current. The separation of these two currents may be achieved in the time domain by making the charging current transient much faster than the signal sampling. However, since the stone amount of charge is required to charge the electrodes to any bias voltage, reducing the charging time causes an increase in the charging current. This increasing current can cause problems through radiated power, output resistance of power supplies or switches, and the saturation of signal amplifiers. These effects can compromise the degree of separation that could be achieved between these currents. The net result is that the degree of accuracy and the ability to operate in a real time environment under high MeV conditions is limited. Thus, it is desirable to provide a liquid-filled ionization chamber which can operate accurately in such conditions. Further, it would be desirable to provide such a chamber without requiring more complex monitoring and amplification electronics.
Accordingly, an ionization chamber which allows an increase in the resolution of images obtained during real time imaging procedures is needed. Preferably, the chamber should be capable of operating with high, photon limited, signal to noise ratios, and other performance characteristics making it suited to dosimetry applications.